Download Maximum bite force, muscle efficiency and mechanical advantage in

 2003 European Orthodontic Society
European Journal of Orthodontics 25 (2003) 265–272
Maximum bite force, muscle efficiency and mechanical
advantage in children with vertical growth patterns
Patricia García-Morales*, Peter H. Buschang*, Gaylord S. Throckmorton**
and Jeryl D. English***
*Department of Orthodontics, Baylor College of Dentistry, The Texas A&M University System Health Science
Center, Dallas, **Department of Cell and Molecular Biology, Southwest Medical University, Dallas and
***Department of Orthodontics, University of Texas Health Science Center, Houston, USA
SUMMARY This study correlated maximum bite force and masticatory muscle electromyography (EMG)
activity with craniofacial morphology and mechanical advantage of children with vertical growth
patterns. From lateral cephalograms of 30 females and 17 males (9.3 ± 3.6 years of age), 13 morphological and eight biomechanical measurements were recorded. Two maximum bite forces and
12 submaximal bite forces along with their associated EMG muscle activity were recorded at the right
mandibular first molar. Muscle efficiency was evaluated using the relationship between bite forces and
EMG activity levels.
There were no significant sex differences (P > 0.05) for any of the morphological, functional or
biomechanical variables. Factor analyses reduced: (1) the 13 morphological variables to four factors
explaining 82.8 per cent of the morphological variance; (2) six functional variables to two factors
explaining 68.8 per cent of the functional variance; and (3) 11 biomechanical variables to three
factors explaining 90.9 per cent of the biomechanical variance. The vertical size factor was negatively
correlated with the muscle efficiency factor (r = –0.39; P = 0.006) and positively correlated with the
moment arm factor (r = 0.67; P < 0.001). The morphological divergence factor was negatively correlated
with the bite force factor (r = –0.34; P = 0.019) and the mechanical advantage factor (r = –0.32; P = 0.028).
The muscle efficiency factor (functional) was negatively correlated with the moment arm factor
(r = –0.33; P = 0.023).
It is concluded that: (1) independent of chronological age, children with larger faces have larger
moment arms and require less muscle activity to attain any given force, and (2) greater hyperdivergence
is related to poorer mechanical advantage and lower maximum bite force. These data support the
relationships between bite force, muscle strength and morphology in children, similar to those reported
for adults.
Introduction
It is generally accepted that a relationship exists between
the form and function of the craniofacial skeleton.
Weaker maximum bite forces have been related to
increased malocclusions, especially in subjects with
open bite tendencies and posterior crossbites with
narrow maxillary arches, and incisor crowding (Bakke
and Michler, 1991; Ellis et al., 1996; Buschang and
Throckmorton, 1997). Even stronger relationships exist
between skeletal hyperdivergence and masticatory
function, including reduced jaw muscle size, lower
maximum bite force, lower electromyography (EMG)
activity and reduced muscle efficiency (Proffit et al.,
1983; Ueda et al., 1998; Granger et al., 1999;
Throckmorton et al., 2000). Although well supported for
adults and children with neuromuscular disease, the
relationship between craniofacial morphology and
function in children without any neuromuscular disease
remains controversial. Proffit and Fields (1983) found
no differences in maximum molar bite forces between
children with high and low mandibular plane angles.
Ingervall and Minder (1997) reported significant
relationships between the maximum bite force and the
mandibular plane angle for girls but not for boys, while
Kiliaridis et al. (1993) showed only weak relationships
between craniofacial morphology and the maximum
incisal bite force, and no correlation with the maximum
molar bite force, in children 7–13 years of age.
The relationships for children may be confounded by
the measures used to quantify muscle function. First,
potential fear, discomfort and pain may make it more
difficult to obtain maximum bite forces in young
children than in adults. Second, bite force by itself is not
adequate to evaluate muscle strength because bite force
is strongly influenced by the amount of voluntary effort,
which may be less than maximal effort. True muscle
strength depends upon muscle size, muscle recruitment,
and the length of the muscle moment arms. Therefore,
the relationship between EMG and bite force, as well as
266
the mechanical advantage of the jaw muscles, should
be determined when assessing jaw muscle strength. The
purpose of this study was to correlate the maximum
bite force and EMG activity of the jaw abductor
muscles (masseter and temporalis) with the craniofacial
morphology and mechanical advantage of children with
a vertical growth pattern as evaluated on lateral
cephalograms. The effects of gender and age on these
relationships were also analysed.
Subjects and methods
Sample description
Forty-seven (30 females and 17 males) growing patients
diagnosed with vertical craniofacial growth patterns
(MP/FH > 32 degrees) were evaluated. They represented
pre-treatment orthodontic patients aged 9.3 ± 2.3 years
(range 7–13 years).
Craniofacial analysis
Standard lateral cephalograms were traced and digitized
(Figure 1) by a single investigator (PGM) using the
Dentofacial Planner Software® (Dentofacial Software,
Inc., Toronto, Canada). The right and left side structures
were traced and averaged for landmark identification.
To assess the reliability of the cephalometric landmark
identification and digitization, a sample of 13 cephalograms was re-traced and re-digitized. Systematic and
method errors were calculated. There was no significant
Figure 1 Cephalometric landmarks and abbreviations. 1, nasion
(N); 2, orbitale (Or); 3, anterior nasal spine (ANS); 4, gnathion
(Gn); 5, menton (Me); 6, posterior symphysis (Sym); 7, gonion (Go);
8, articulare (Ar); 9, condylion (Co); 10, coronoid (Cor);
11, posterior nasal spine (PNS); 12, sella (S); 13, pterygomaxillary
fissure (PTM); 14, lower first molar (L6); 15, lower central incisor
(L1); 16, SN midpoint (MidPt).
P. G A R C Í A - M O R A L E S E T A L .
systematic error, and the method error ranged between
0.19 and 1.33 mm, with the condyle to the lower first
molar measurement having the highest error.
Morphological variables
Morphological measurements included six linear and
seven angular parameters (Table 1). Linear distances
from nasion to anterior nasal spine and anterior nasal
spine to the menton were included to calculate upper
and lower face height, total anterior face height and the
ratio of lower to total anterior face height. The distance
from the sella to gonion indicated posterior face height.
The vertical heights of the lower incisor and lower first
molar to the mandibular plane and symphysis width
were also recorded. Angular parameters included the
deflection from (1) the sella–nasion line (cranial base) to
the palatal, occlusal, and mandibular planes; (2) the
y-axis and (3) the three angles of mandibular position
(posterior angles): nasion–sella–articulare, sella–articulare–
gonion and articulare–gonion–menton.
Biomechanical variables
The biomechanical variables were defined as described
by Throckmorton and Dean (1994). Eight biomechanical
parameters were included. The muscle vector direction
(Figure 2) of the superficial masseter, medial pterygoid
and anterior temporalis was determined by the graphic
landmarks representing the origin and insertion of these
muscles. The superficial masseter was represented by a
line from the gonion to orbitale, the medial pterygoid by
a line from the gonion to the pterygomaxillary fissure,
and the anterior temporalis by a line from the coronoid
tip to the midpoint of the sella–nasion plane. The other
five biomechanical variables were the dental and muscle
moment arms, which were used to calculate the mechanical advantage of each muscle. The dental moment
arms included the incisor arm, (the distance parallel to
the occlusal plane from the tip of the lower incisor to the
condylar summit, defined by the cephalometric landmark condylion) and the molar arm (measured the same
way from the tip of the mesiobuccal cusp to the condylar
summit). The three muscle moment arms were the
distances perpendicular to the condylar summit from
each muscle vector. The mechanical advantage of each
muscle was the ratio between its muscle moment arm
length and the dental moment arm length. For each
muscle, moment arms were calculated for the incisor
and the molar bite positions.
Maximum bite force (functional variables)
Bite force was measured using a unidirectional
transducer placed between the upper and lower right
first molars. The metal arms of the transducer were
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M U S C L E F U N C T I O N I N V E RT I CA L G ROW E R S
Table 1
Standard morphological and functional variables and their mean values.
Mean
Morphological variables
N–ANS (mm)
ANS–Me (mm)
S–Go (mm)
PP–SN (º)
OP–SN (º)
MP–SN (º)
N–S–Ar (º)
S–Ar–Go (º)
Ar–Go–Me (º)
y-axis (º)
L1⊥MP (mm)
L6⊥MP (mm)
Pg–Sym (mm)
Functional variables
Bite force (N)
Right masseter slope
Left masseter slope
Right temporalis slope
Left temporalis slope
Standard deviation
51.37
70.44
70.88
3.69
25.16
40.33
124.01
142.67
133.65
70.36
40.05
30.47
15.4
4.03
5.46
4.98
3.45
5.34
5.67
4.59
5.72
5.51
4.18
3.61
2.95
1.71
379.05
1.704
1.864
1.807
1.525
129.53
0.806
0.873
0.772
0.725
Minimum
Maximum
37.7
61.3
57.4
–4
12.8
29
113.1
132
122.3
61.3
33.9
25.1
11.6
62.6
88.6
80.2
12
37.7
52.4
135.7
158
145.2
80.8
55.3
40.6
19.4
96.96
0.186
0.540
0.717
0.469
550.41
4.314
4.165
4.443
4.937
forces were recorded at the right mandibular first molar.
In addition, five submaximal force levels (approximately
10, 20, 40, 60 and 80 per cent of maximal bite force)
were recorded. Occlusal force levels during isometric
bite were controlled by displaying the subject’s force
output on an oscilloscope. In addition, a 1 second recording of muscle activity was taken at a standard bite force
level of 112 N. These seven recordings were repeated,
giving a total of 14 bite force recordings.
EMG recordings
Figure 2 Muscle (•) and bite ( ) moment arms used to calculate
mechanical advantage. 1, molar moment arm (L6–Coocclusal
plane); 2, incisor moment arm (L1–Coocclusal plane); 3, medial
pterygoid muscle [Co⊥(PTM–Go)]; 4, anterior temporalis muscle
moment arm [Co⊥(MidPt–Cor)]; 5, superior masseter muscle
moment arm [Co⊥(Or–Go)].
●
covered with polypropylene tubing to protect the teeth.
Voltage changes to the transducer were amplified
820 times (model 2310 strain gauge conditioner amplifier,
Measurements Group, Inc., Raleigh, North Carolina,
USA) and were digitized at a sampling rate of 4000 Hz
using Optotrak® software (Northern Digital, Corp,
Waterloo, Canada). The root mean squared voltage was
averaged over a 1 second sample of each isometric bite
to determine the force level for that bite. Maximum bite
Muscle activity was recorded bilaterally from the
anterior temporalis and superficial masseter muscles
using bipolar surface electrodes. The electrode sites
were scrubbed with alcohol to reduce skin resistance
and pairs of self-adhesive disposable pre-gelled
Ag/AgCl electrodes (Cardiotrace, Kendall, Chicopee,
Massachusetts, USA) were placed on the skin over the
masseter and anterior temporalis muscles bilaterally.
The position of the masseter origin was palpated while
the subject clenched their teeth. The superior edge of
one electrode pad was placed along the inferior border
of the zygomatic arch, centred over the masseter origin.
A second electrode pad was placed with its superior
edge directly against the inferior edge of the first
electrode pad, resulting in an inter-electrode distance of
1.0 cm. For the temporalis muscle, the inferior edge of
one electrode pad was placed against the palpated
superior border of the zygomatic arch directly above the
masseter electrodes. A second temporalis electrode pad
was placed with its inferior edge directly against the
superior edge of the first temporalis electrode pad,
again resulting in an inter-electrode distance of 1.0 cm.
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Electrode impedance was not measured. However,
the level of EMG output for each pair of electrodes
was observed in real time while the patient performed
maximum clenching. Any electrodes whose output
deviated significantly from the other three electrodes
were replaced until the signals from all four electrodes
were similar. A ground electrode was placed on the
neck. Signals from these muscles were amplified by 1000
using a set of differential amplifiers (Grass Model 511j,
Quincy, Massachusetts, USA) with a band pass of
30–1000 Hz and were digitized in real time at a sampling
rate of 4000 Hz per channel using Optotrak® software.
The quality of the signals was monitored in real time on
a computer screen. The 14 muscle signals and associated
bite force levels were recorded and stored on the
computer hard disk.
P. G A R C Í A - M O R A L E S E T A L .
were evaluated using independent t-tests. To increase
reliability and decrease the number of tests, principal
component analyses with varimax rotation were performed to evaluate the multivariate patterns of
association between the variables. Principal component
factor analyses extract latent composite variables based
on their ability to identify the effects of higher order
correlations and determine priorities between variables
and groups of variables (Thorndike, 1978; Throckmorton
et al., 2000). Independent sets of factors were estimated
for the morphological, functional, and biomechanical
variables. Factor scores were computed for each subject.
Relationships between the morphological, functional,
and biomechanical factors were statistically analysed
using Pearson product-moment correlations.
Results
Jaw muscle efficiency
Although it is not possible to measure directly the force
used by each jaw muscle to generate isometric bite, it is
possible to estimate the relative efficiency of force
development by comparing EMG levels per unit of bite
force between individuals or groups (DeVries, 1968).
Muscle efficiency was evaluated based on the relationship between bite force (N) and EMG (9 volts)
amplitude, for which there is a linear or near linear
relationship (DeVries, 1968; Hagberg et al., 1985;
Lindauer et al., 1991; Throckmorton and Dean, 1994).
For the purpose of this research, muscle ‘efficiency’ was
defined as the amount of electrical activity used per unit
of bite force. Some caution is necessary when defining
efficiency this way because, by this definition, muscles
that do not contribute to the generation of bite force
would appear to have maximum ‘efficiency’. However,
the two muscles examined in this study have been
shown to contribute to maximum bite force (DeVries,
1968; Bakke et al., 1989; Lindauer et al., 1991; Tate et al.,
1994). In addition, efficiency was compared only within
the same muscle between groups, not between muscles.
The slope relating EMG activity to biting force measures
how efficiently a muscle can generate force. For each
muscle, a least square regression was fitted to the 14 bite
force values (two maximum, two constant, 10 submaximum) and associated EMG levels. In most cases of
regression, coefficients (r2) between EMG and bite
force were greater than 0.9 for each muscle within each
individual, and the slopes were significantly different
from zero. The slope of the least square regression
(EMG/force slope) indicates the amount of effort a
muscle uses to generate an isometric force.
Statistical analysis
Skewness and kurtosis statistics showed that the
variables were normally distributed. Sex differences
Because the t-tests showed no significant sex differences
(P > 0.05) for any of the morphological, functional or
biomechanical variables, the sexes were pooled for
subsequent analyses. Tables 1 and 2 provide descriptive
statistics for the morphological, functional and biomechanical variables. Morphological and functional
factors are shown in Table 3 and biomechanical
factors in Table 4. Factor analyses were used to reduce
the 13 morphological variables to four factors explaining 82.8 per cent of the morphological variance, six
functional variables to two factors explaining 68.8 per
cent of the functional variance, and 11 biomechanical
variables to three factors explaining 90.9 per cent of the
biomechanical variance.
Morphological factors
The first morphological factor, explaining 36.0 per
cent of the morphological variance, was referred to
as ‘vertical’, based on the five variables that defined this
factor (Table 3). Children with larger vertical values
had larger lower anterior and posterior face heights,
increased anterior and posterior mandibular dentoalveolar heights and greater symphysis width, suggesting
that vertical is basically a size factor. The second
morphological factor, explaining 24.3 per cent of the
variance, was referred to as ‘divergence’. Children with
larger divergence values had steeper occlusal and
mandibular plane angles and a larger y-axis. Children
with larger upper face values, the third factor explaining
13.5 per cent of the morphological variance, had a
larger upper face height and an increased palatal
plane to cranial base angle. The last morphological
factor, explaining 8.8 per cent of the variance, was
referred to as the ‘posterior angles’ factor. Children
with larger posterior angles values had larger cranial
base angles, smaller articular angles and larger gonial
angles.
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M U S C L E F U N C T I O N I N V E RT I CA L G ROW E R S
Table 2
Biomechanical variables, mean values (right and left sides averaged), and definitions.
Mean
Muscle force directions
Mas Dir
MPtery Dir
ATemp Dir
Moment arms
Incisor Arm
57.21 ± 4.67º
80.42 ± 5.28º
99.98 ± 7.65º
92.28 ± 6.50 mm
Molar Arm
61.88 ± 5.66 mm
Sup Mas Arm
41.37 ± 2.81 mm
Med Ptery Arm
27.20 ± 2.40 mm
Ant Temp Arm
29.76 ± 5.61 mm
Mechanical advantage
Mas/Incisor Adv
Mas/Molar Adv
MPtery/Incisor Adv
MPtery/Molar Adv
ATemp/Incisor Adv
ATemp/Molar Adv
0.44 ± 0.02
0.67 ± 0.05
0.29 ± 0.02
0.44 ± 0.03
0.32 ± 0.05
0.48 ± 0.08
Definition
Superficial masseter direction: line joining gonion and orbitale
Medial pterygoid direction: line joining gonion and the postero-superior point on
the pterygomaxillary fissure
Anterior temporalis direction: line joining the coronoid tip and the midpoint of
the sella–nasion line
Perpendicular distance from L1 incisal edge to the condylar summit along a line
parallel to the functional occlusal plane
Perpendicular distance from L6 mesiobuccal cusp to the condylar summit along a
line parallel to the functional occlusal plane
Superficial masseter moment arm: perpendicular distance from SMas Dir to the
condylar summit
Medial pterygoid moment arm: perpendicular distance from MPtery Dir to the
condylar summit
Anterior temporalis moment arm: perpendicular distance from ATemp Dir to
the condylar summit
Ratio of Sup Mas Arm/Incisor Arm
Ratio of Sup Mas Arm/Molar Arm
Ratio of Med Ptery Arm/Incisor Arm
Ratio of Med Ptery Arm/Molar Arm
Ratio of Ant Temp Arm/Incisor Arm
Ratio of Ant Temp Arm/Molar Arm
Table 3 Morphological and functional factors.
Table 4 Biomechanical factors.
Morphological factors
1. Mechanical advantage (34.80 per cent)
MAMas/I
MAMPtery/I
MAMas/M
MAMPtery/M
2. Moment arms (33.35 per cent)
Inc Arm
SupMas Arm
Molar Arm
MPtery Arm
3. Anterior temporalis (22.75 per cent)
MAATemp/I
ATemp Arm
MAATemp/M
Vertical size (36.01 per cent)
ANS–Me
L1⊥MP
S–Go
L6⊥MP
Symphysis
Divergent (24.34 per cent)
OP–SN
y-axis
MP–SN
Upper face (13.54 per cent)
N–ANS
PP–SN
Posterior angles (8.84 per cent)
N–S–Ar
Ar–Go–Me
S–Ar–Go
Functional bite force (37.52 per cent)
MBF 1
MBF 2
Muscle efficiency (31.27 per cent)
Right masseter slope
Right temporalis slope
Left masseter slope
Left temporalis slope
+0.879
+0.894
+0.729
+0.882
+0.671
+0.853
+0.689
+0.874
+0.809
+0.816
–0.461
–0.679
+0.945
+0.929
+0.911
+0.813
+0.751
+0.782
+0.424
+0.871
+0.866
+0.915
+0.785
+0.856
+0.828
+0.883
+0.902
–0.970
+0.925
–0.983
second factor, explaining 31.3 per cent of the functional
variance, was referred to as ‘muscle efficiency’ and
contained the EMG/force slopes of the right and left
masseter and temporalis muscles. Higher values for
bite force and muscle efficiency indicated higher bite
forces and more muscle effort per unit of bite force,
respectively.
Biomechanical factors
Functional factors
The first functional factor, explaining 37.6 per cent of
the functional variance, was referred to as ‘bite force’
because it included the two maximum bite forces. The
The first biomechanical factor, explaining 34.8 per cent
of the variance, was referred to as the ‘mechanical
advantage’ factor because it contained the mechanical
advantage of the masseter and medial pterygoid muscles.
The greater the molar and incisal mechanical advantage
of these muscles, the greater the mechanical advantage
270
P. G A R C Í A - M O R A L E S E T A L .
factor value. The second factor, explaining 33.4 per cent
of the biomechanical variance, was referred to as
‘moment arms’. Children with larger moment arms
factor values had larger incisor, molar, masseter and
medial pterygoid moment arms. The last biomechanical
factor, explaining 22.8 per cent of the biomechanical
variance, was referred to as ‘anterior temporalis muscle’.
Children with greater values for the anterior temporalis
muscle factor had larger moment arms and less
mechanical advantage for both the molar and incisor
bites.
Correlations among factors
Correlations among factors are shown in Table 5. Two
of the morphological factors were significantly related
to two of the functional factors and two of the
biomechanical factors. The vertical size factor was
negatively correlated with the muscle efficiency factor
(r = –0.39; P = 0.006) and positively correlated with the
moment arms factor (r = 0.67; P < 0.001), indicating that
individuals with larger vertical size values had larger
moment arms and required less muscle activity to attain
any given bite force. Analyses of covariance, controlling
for chronological age, showed similar correlations
for the muscle efficiency factor (r = –0.30; P = 0.047)
and the moment arms factor (r = 0.62; P < 0.001).
The divergence factor was negatively correlated with
the bite force (r = –0.34; P = 0.019) and mechanical
advantage factors (r = –0.32; P = 0.028). This indicates
that individuals with larger divergence values had less
mechanical advantage, which is also reflected in less bite
force. The upper face and posterior angles factors were
not correlated with any functional or biomechanical
factors. The muscle efficiency factor was negatively
correlated with the moment arms factor (r = –0.33;
P = 0.023), indicating that individuals with longer
moment arms needed less muscular activity (smaller
EMG/force slope) to attain their maximum bite forces.
Discussion
A relationship between weak jaw muscles, malocclusion
and skeletal deformity has been well established for
children with neuromuscular disease. Numerous studies
have shown that children with Duchenne and myotonic
dystrophy have a high prevalence of malocclusion,
narrow and deep palates, increased face heights, more
obtuse gonial angles, and steeper mandibular plane
angles that are associated with weaker bite forces and
less efficient jaw muscles (Hamada and Kawazoe, 1977;
Kiliaridis et al., 1989). In fact, Kiliaridis et al. (1989)
suggested that vertical aberration of craniofacial growth
might be one of the first signs of this disease. Similar
relationships have been reported for children with spinal
muscular atrophy (SMA). Compared with matched
controls, SMA subjects fatigue more easily and have
43 per cent less maximum bite force and 44 per cent less
efficient masticatory muscles (Granger et al., 1999).
Houston et al. (1994a) found that 60 per cent of SMA
patients presented with malocclusions for which treatment
was mandatory; these subjects have a high prevalence of
Class II malocclusions, excessive overjet, open bite,
crowding and maxillary transverse hypoplasia. Untreated
SMA subjects also show skeletal hyperdivergence,
accentuated in the lower anterior third of the face, with
a moderate tendency for a protrusive maxilla and a
retrognathic mandible (Houston et al., 1994b).
Relationships between weak muscles, malocclusion,
and hyperdivergence have also been well established for
adults (Ringqvist, 1973; Ingervall and Helkimo, 1978;
Proffit et al., 1983; Bakke and Michler, 1991; Raadsheer
et al., 1999; Kayukawa, 1992; Bennington et al., 1999;
Throckmorton et al., 2000). Kiliaridis et al. (1993)
reported no relationship between maximum molar bite
forces and anterior face height, as measured from facial
photographs. However, anterior face height by itself is a
better indicator of facial size than skeletal divergence.
Tuxen et al. (1999) were also unable to demonstrate a
Table 5 Correlations between morphological, functional and biomechanical factors.
Bite force
Vertical size
r
P
Divergence
r
P
Upper face
r
P
Posterior angles
r
P
Muscle efficiency
Mechanical advantage
–0.397**
0.006
–0.105
0.484
–0.340**
0.019
–0.003
0.986
–0.321**
0.028
–0.145
0.332
–0.160
0.282
0.096
0.520
–0.141
0.345
0.110
0.460
0.201
0.175
0.123
0.411
0.150
0.315
–0.118
0.430
–0.008
0.960
–0.224
0.129
0.102
0.494
0.085
0.570
**Correlation significant at the 0.001 level; *correlation significant at the 0.05 level.
Moment arms
0.675**
0.000
Anterior temporalis
–0.095
0.525
271
M U S C L E F U N C T I O N I N V E RT I CA L G ROW E R S
correlation between maximum molar bite force and
facial morphology, but this may be due to their small
homogeneous sample. Skeletal divergence has also been
related to muscle volume and thickness (Gionhaku and
Lowe, 1989). Theoretical models indicate a negative
relationship between mechanical advantage and divergent craniofacial morphology (Haskell et al., 1986) that
has been verified for adult patients (Throckmorton
et al., 1980; Ferrario et al., 1999).
The relationship between divergent morphology and
EMG muscle activity levels remains controversial.
Miralles et al. (1991) compared the EMG activity of
Class I, II, and III malocclusion groups, and found no
differences at maximum clenching but higher muscular
activity in Class III patients in the postural position
and during swallowing. Ueda et al. (1998) performed a
study where EMG activity of the masticatory muscles
(masseter, temporalis and digastric muscles) was evaluated
during the day with a portable EMG recording system.
They found a relationship between their measurements
of skeletal divergence (mandibular plane angle and the
ratios between anterior to posterior total and lower face
height), and muscle activity, but no sex differences in
EMG muscular activity. Fogle and Glaros (1995) evaluated
EMG activity at rest and found no relationship with
craniofacial form, but a significant canonical correlation
between EMG activity and the mandibular plane angle.
Interpretation of these results in terms of muscle strength
is problematic because none simultaneously recorded
bite force.
The present results show a relationship between
skeletal divergence and maximum molar bite forces in
children aged 7–13 years. Other studies have reported
no such relationship. Proffit and Fields (1983) found no
correlation between maximum molar bite force and the
mandibular plane angle for children aged 6–11 years.
However, maximum bite forces tended to be lower in
the 12 high-angle children, and a post-hoc power
analysis indicated that their sample was too small to rule
out a difference. Ingervall and Minder (1997) found a
significant correlation between maximum molar bite
force and the mandibular plane and gonial angles for
66 girls aged 7–16 years, but not for 54 boys aged 8–15
years. No significant differences in the relationships
between boys and girls were found in the present study,
but this may reflect the smaller sample size or the
younger ages of the subjects. Kiliaridis et al. (1993)
found only a weak relationship between maximum
incisor bite force and the ratio of upper and lower face
height in 99 children aged 7–13 years. They reported
significant differences in face height ratios between
boys and girls at these ages, but no significant
differences in maximum molar bite force. They also
found no association between face height ratios and
maximum molar bite force. However, they averaged
maximum molar bite forces over a 10 second recording
period during molar biting. This relatively long interval may have resulted in less reliable maximum bite
forces.
Two other factors make it difficult to establish a
relationship between morphology and maximum bite
force in children. First, young children have more
difficulty following instructions and may be less likely to
make a maximum effort during biting tasks. Proffit and
Fields (1983) reported considerably higher variance in
the maximum bite force generated by children than
adults. For this reason, multiple recordings of maximum
bite force might be more reliable than a single
recording. Second, many studies use correlations of
single morphological measures rather than multivariate
factors. The use of multivariate factors to define
craniofacial morphology might be expected to provide a
more reliable measure of divergence than any single
variable. In addition, the use of a sample composed
entirely of children diagnosed with divergent growth
patterns may have resulted in larger ranges of morphological and functional variation, resulting in stronger
correlations.
The more divergent children not only had weaker
maximum bite forces, but they also had poorer
mechanical advantage. Throckmorton et al. (2000)
showed a similar relationship between hyperdivergence
and mechanical advantage of the masseter and medial
pterygoid muscles in adults. Theoretically, muscles
with poorer mechanical advantage must generate more
tension to produce a bite force equivalent to that
produced by muscles with better mechanical advantage. Hyperdivergent individuals tend to have poorer
mechanical advantage, particularly of the masseter
muscle (Throckmorton et al., 1980; Haskell et al., 1986),
suggesting that they should have a lower maximum bite
force. However, experimental studies suggest that
mechanical advantage makes only a small contribution
to maximum bite force compared with other factors
(Throckmorton and Dean, 1994; Throckmorton et al.,
2000).
Conclusions
In general, this study shows that relationships between
bite force, muscle strength and morphology in children
are similar to those reported for adults. Specifically, it
was found that:
1. Independent of chronological age, larger children
have larger moment arms and require less muscle
activity to attain any given force.
2. Greater hyperdivergence is related to poorer
mechanical advantage and lower maximum bite
force.
272
Address for correspondence
Peter H. Buschang
Department of Orthodontics
Baylor College of Dentistry
Texas A&M University System
3303 Gaston Avenue
Dallas, TX 75246
USA
Acknowledgement
This research was partially supported by an AAOF
Center Grant.
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